1. Field of the Invention
This invention relates broadly to methods of coating transparent articles. In particular, this invention relates to the method of producing copper-silver films by chemical replacement by silver of copper in a conventionally deposited copper film.
2. Description of the Prior Art
Transparent articles bearing a transparent metal film have been produced by various well-known electroless or wet chemical coating or plating tehcniques. These techniques generally involve the contacting of the article to be coated with suitable solutions to deposit a metal film thereon by reducing the metal from a metal salt in the solution.
Autocatalytic coating techniques are known wherein a reducing agent such as an alkali metal hypophosphite or formaldehyde is present in a single solution with the metal salt. Such a solution is not usually susceptible to rapid reaction until it is placed in contact with a catalytic or sensitized surface. Autocatalytic techniques for the deposition of cobalt are described in U.S. Pat. No. 2,532,283 and 2,532,284, to Brenner and Riddell. Other techniques of this type are described in patents such as U.S. Pat. No. 2,956,900 to Carlson et al., relating to the deposition of nickel.
Exhaustive electroless coating techniques are also known. In these techniques the reduction reaction will proceed rapidly once the reducing agent is present together with the metal salt-containing solution. For this reason, such techniques usually require applying separate solutions of these reactants substantially simultaneously to the substrate to be coated. Among such techniques are those wherein a boron-containing reducing agent is employed, such as described in U.S. Pat. No. 3,671,291 to Miller and Cavitt for the deposition of iron, nickel and cobalt.
The reflectance and transmittance characteristics of glass may also be modified by introducing metals into the surface of the glass itself. Typical staining techniques for accomplishing this result are described in U.S. Pat. No. 2,701,215 to Kroecl and U.S. Pat. No. 3,079,264 to Grego et al. Such techniques are not, however, coating techniques and do not result in a metal film on the surface of the substrate. Nor are such techniques replacement reactions since there is no metal film involved to be replaced.
While the practice of plating bulk metals presents substantially different problems from the coating of transparent articles with thin, transparent reflective films, immersion plating techniques for such purposes are well known. In the METAL FINISHING GUIDEBOOK for 1965, at page 407, there is described a method for plating silver over copper metal. The method comprises contacting a copper article with a solution containing 1 ounce-per-gallon of silver nitrate, 10 ounces-per-gallon ammonia, and fourteen ounces-per-gallon of sodium thiosulfate.
The preparation of thin, transparent films on transparent substrates, particularly on large sheets of glass for architectural use, presents particular problems which are not present in the coating of opaque articles, particularly opaque metallic articles. In the making of large, transparent, coated articles for architectural use, it is of extreme importance that films of uniform thickness, transmittance and reflectivity be produced in order for the articles to have a uniform aesthetic appearance. Films of the requisite uniformity have been prepared comprising various metals including iron, cobalt, nickel, copper, silver and mixtures thereof, by direct deposition onto the substrate utilizing electroless coating techniques.
Patented electroless coating techniques have been employed successfully in producing highly uniform films having an aesthetically pleasing appearance and a high reflectivity for energy in the infrared range. Articles coated in accordance with these techniques are used in buildings to provide attractive windows or curtain walls which effectively reflect solar energy thus minimizing the load imposed on air conditioning systems servicing these buildings.
Commercial architectural products such as described above have been produced according to the teachings of U.S. Pat. No. 3,457,138 to Miller, which describes a method of making highly efficient infrared reflective films by the direct deposition of a copper film over a directly-deposited silver film. Such films are rich in copper and have a pleasing pink appearance.
A method of producing articles having other desirable color characteristics and solar energy control capabilities is disclosed in the copending application of Charles B. Greenberg and Peter P. Harmon, U.S. Ser. No. 384,763 now U.S. Pat. No. 3,920,864. This method requires a thiosulfate complexing agent and provides a means for producing films having lower reflectivity and an aesthetically desirable brown-gray appearance. Such films, however, are not essentially metallic, rather having most of the copper in an oxidized state as an oxide or perhaps a sulfide.